It should be noted that the explosion occurs because of the sudden
evolution of the SO2 gas.

On the basis of the stoichiometry presented, the reaction mixture
should be 58.7% KClO3, 34.9% Sb2S3, and 6.4% P4.
The actual reaction mixture is 63% KClO3, 31% Sb2S3,
and 6% P4, which means there is a slight excess of both potassium chlorate and
phosphorus.